Aircraft Icing, Causes

The combination of flying aircraft and icing conditions can be bad news. How beautiful ice and snow may be in winter time, for aircraft it can spell havoc. Aircraft depend on the airflow over their wings and any amount of ice disrupts that, increasing weight and more importantly: lift is affected.

For general aviation and homebuilt aircraft pilots its important to remove any trace of snow and ice from the aircraft if it has been parked outside during freezing winter nights or after a snow shower. Recognizing where icing conditions can develop is very important for all pilots.

When ice develops

If you see any ice developing on your aircraft get yourself out of these conditions at once. Unless its freezing rain or drizzle (which would require immediate and prompt action) there is no reason to panic, just keep calm.

Type of aircraft ice

There are a couple of types important for the pilot: rime, clear or glaze and mixed (which is a combination). Frost is also formed when moist air contacts a subzero (0 °C or 32 °F) surface, if the temperatures remain above freezing we call it dew. Outside overnight parked aircraft can develop frost in the morning when the conditions are right (moisture, no wind and cooling below dewpoint).

Rime ice

This is a milky white appearance following the contours of the surface it forms on. Its brittle and relatively easy to remove for aircraft anti and deice systems.

Clear or Glaze ice

Transparent compared to rime ice. Has a clear and smooth surface (think ice cubes), can have air pockets and initially follows the surface but with higher accretion it becomes lumpy. More difficult to remove.

Effects of ice

There are a number of effects: the change in wing geometry distorts the airflow, reducing lifts, angle of attack for maximum lift, increases weight and drag.

The tricky part with icing is that only a small amount is needed to reduce lift in significant amounts. Wind tunnel and flight tests have shown that ice similar to coarse sandpaper can reduce lift by 30% and increase drag up to 40%.

Where ice develops

For ice you need subzero temperatures and moisture. Visible moisture can be in the form of clouds, and some cloud type have more moisture than others. Cumuliform and stratiform clouds between 0 and minus 15°C, carry a high risk of ice. But, the colder it gets, the less moisture the air can hold and so the risk of ice below 20° reduces rapidly with dropping temperatures.

Frontal systems

Fronts with low pressure areas and instable air masses are capable of producing large amounts of ice due to the higher water content. With drizzle and freezing rain, where the droplet is still liquid with temperatures below zero, the risk is the greatest. These conditions promote rapidly increasing clear ice, making flights dangerous.

This situation is seen with rapidly moving cold fronts lifting warm moist air to above the freezing level. The air condensates and rain can fall. The area beneath the warm air is cold and when rain falls from the lifted and saturated clouds it cools and when the droplet hits the cold airframe it freezes and forms clear ice.

The thing is to avoid these frontal areas. Just remain VFR and when flying below lifted clouds or moist air (after cold front passage) do keep an eye out on icing conditions when precipitation starts.

Staying clear

For VFR pilots its rather easy: stay clear from clouds. And make sure the carburetor and pitot heat is functional. For IFR pilots it becomes a challenge: get PIREPs and WX reports, check where the fronts are and the direction they are moving, note cloud tops as you may not have the capability to go 'on top', check cloud base and freezing levels to see where the warm air is. Are they frontal clouds (larger area) or air mass clouds? Make sure to have an alternative and escape route should it become necessary.